Phone gets lag in low battery and once plug in charger it gets better

I have an iphone 6s and i have been using for around 1 year perfectly. But now it gets so much lag under low battery and when it is connected to the charger it goes off and works smoothly. And also i had upgraded to 10.3.3 version n thought this might be the reason. Any tips or suggestions.??

Welcome to iMore Johnny and THANKS for joining. Have you tried a hard reboot. Hold down the power button and down volume button at the same time until you see the Apple () logo and release. Allow the iPhone to reboot. See if that helps.